This invention relates to an efficacious and safe method of controlling rodent population. Rodents, especially Norway rats commmon to the United States, roof rats, black rats, ground squirrels, prairie dogs, mountain beaver, etc., feed on a wide variety of foods. Rats, in particular, do a trememdous amount of damage by their eating habits, destroying agricultural crops, stored grain and stored produce, as well as contributing to the spread of disease through their infestation of urban areas. Attempts to control or eliminate rodents have been made for many years. Because the rodents quickly learn to avoid the poisons and poison impregnated foods, the use of poisoned food or baits as practiced to date does not provide a long term solution to the rodent problem.
The need for economical rodent control methods for agricultural, commensal and industrial uses, necessitates a material that can be easily dispersed in the fields or in buildings and provide an efficacious long term method of rodent control. Many of the baits depend upon the impregnation of seeds, fruits, peanut butter, etc. with a poison. The rodents quickly learn to avoid such poison bait by the odor or taste, particularly when the discomfort of the ingestion of a sub-lethal quantity is associated with the bait. Therefore, it is advantageous to prepare a poison that has its odor and taste effectively masked and the onset of the reaction to the poison is delayed until a lethal amount is consumed and the rodent has left the bait area.
Attempts to encapsulate rodenticides have been made but generally enough rodenticide is entrapped in the encapsulating material or will permeate through it to allow the rodent to identify the fact that a poison is present. Also, methods have been developed to prepare enteric coated rodenticides, e.g. U.S. Pat. No. 2,957,804 wherein the rodenticide is surrounded by a resin material that will not dissolve in the acidic conditions of the stomach, but dissolves in the alkaline condition of the rodent's intestines. The theory is that the rodent that has eaten the poison would not realize that it was deadly since there would be considerable time delay from the ingestation to death, and thus would not avoid the enteric coated material. The efficacy of such encapsulated material has been shown to be no more than unencapsulated material because the amount of encapsulant needed to mask the material also decreases its availability as a poison.
Zinc phosphide (Zn.sub.3 P.sub.2) is an efficient rodenticide in an acidic environment because it releases phosphine, a deadly poison. It is effective against many species of rodents. It suffers from the disadvantage that rodents easily identify it in a bait and avoid it. Zinc phosphide is considered one of the safest rodenticides to control rats and mice because of its emetic properties and the fairly rapid dissipation of the poison in the rodent's body. Thus, children, domestic animals, etc. will regurgitate the poison before it is lethal, whereas rodents cannot regurgitate. Zinc phosphide in the body of a dead rodent reacts with the fluids in the rodent's body and is consumed in a way which eliminates the hazard of secondary poisoning of other animals who may eat the dead rodent. For this reason zinc phosphide has been widely used for controlling rodents in agriculture so as to protect rice, sugarcane and other growing plants against attack. In the western United States, for example, zinc phosphide has had wide-spread use to control ground squirrels, and the like.